Titanium foam scaffolds made to match bone’s properties

Research into better medical implants has led researchers to porous titanium …

Developing the ideal structural medical implant is a complicated task, and different applications require different approaches. We have reported on the need for higher strength titanium that allows smaller implants to be made for delicate work, but in some cases, high strength is actually a problem. Researchers have investigated the mechanical properties of engineered porous titanium structures in an effort to tune them to be as similar as bone as possible.

When a large structural medical implant is made, for example, the spike driven into the femur for a hip replacement surgery, its job is to carry load and integrate with the body as tightly as possible. This means complete biocompatibility and resistance to corrosion, both of which titanium excels at. Structurally, the bone must grow and adhere to the implant otherwise it is liable to wiggle around and slip out of place – making for a painful, risky and expensive surgical repair. This has been a problem historically, and progress has been made by using highly textured coatings produced by a variety of means. This is an improvement, but still not ideal. Also, one of the less commonly understood problems is that metallic implants are simply too strong: the higher stiffness relative to the bone means they will carry more of the load, causing the bone to atrophy just like an unused muscle, leading once again, to failure of the implant system.

A metallic foam is an ideal structure for an implant: it retains the corrosion resistance and biocompatibility as one would expect, but an open celled porous structured allows bone to grow through the foam, making for a sturdier interface, while the foam structure could be tailored to precisely match bone’s natural mechanical properties. It’s a wonderful idea, but producing adequate metallic foam is no small task. Researchers continued investigations of foams produced with the relatively new selective electron beam melting (SEBM) additive manufacturing process.

Instead of the traditional metal processing route of casting a large, solid ingot of material and then shaping and removing material to make a shape (which as one would imagine makes producing a foam difficult), this takes the opposite approach and adds material rather than subtracts it. A 3D CAD model of the structure is made, and sliced into hundreds or thousands of 2D cross sections. These cross sections are fed into a machine that spreads a thin layer of powdered titanium on a table, and a beam of electrons, accelerated by tens of thousands of volts, is traced over the features of one of the 2D slices. The electron beam melts the powder, but only locally, allowing for very precise features to be produced. After the tracing of one slice is done, the table drops by the thickness of a slice, and the whole process is repeated.

The unmelted powder acts as support for the structure as its being built, and is later cleaned out of the part, leaving only solid metal behind. Researchers produced several different lattice-like structures of metal, and went about testing them for their elastic modulus (stiffness) and compressive strength. It was shown that a homogenous structure with tunable mechanical properties could be produced; while past studies have shown that material produced by the SEBM process retains its biocompatibility.

With few limits on what structures can be produced by additive manufacturing processes like SEMB, this is a great tool for low volume, high value parts like medical devices. I’d also like to point out that powder metal opens the door for not only cheaper titanium in general, but novel alloys not possible by traditional melt metallurgy. Look for more interesting research using additive manufacturing and powder metallurgy in the future, and the next generation of medical implants using these technologies as well.